Recently, an amount of used fossil fuels, particularly, oil fuels is rapidly increased, seriousness of air pollution due to a pollution source generated by burning the oil fuel rises all over the world. In addition, not only there occur oil spills of large-sized oil tankers but also oil prices are rapidly increased. Accordingly, to avoid harmful influences of the oil fuel, researches on technologies to reduce the amount of used oil fuel have been performed from various angles.
There are automobiles that require the oil fuel. Manufacturers of automobiles have performed various attempts and researches to reduce the amount of used oil fuel. A method of improving fuel efficiency of automobiles, which is one of conventional methods of reducing the amount of used oil fuel, is presently developed and applied. As the method, there is a method of improving combustion efficiency and power transmission efficiency of engines. As another method, there is a method of reducing an amount of energy required in moving in a unit distance by reducing a weight of a car body.
To reduce the weight of a car body, there is a method of replacing parts of the car by lightweight material having a low specific gravity. However, till now, there are little materials replacing superiority of steel products. Accordingly, so far, there are many cases of using steel products as parts of automobile and it is general to try to improve fuel efficiency of an automobile by reducing a weight of the steel products.
When simply reducing a weight of a steel product, since a supportable load is determined for a unit weight, a fatal problem in security of automobiles may be caused. Accordingly, reducing weights of parts may be embodied after solving a problem of manufacturing parts with high strength.
Particularly, a spring for an automobile is a part strongly requiring excellent permanent deformation resistance similar to high strength. The permanent deformation resistance indicates a resistance to a permanent deformation where there is a change in height of a spring used for a long time and incapable of restoring elasticity. To increase the permanent deformation resistance of a spring, steel wire rods where a large amount of Si is added are usually used as materials for springs. Si increases yield strength of steel, thereby preventing permanent deformation.
Also, Si is an element belonging to IV group in a periodic table and acting similarly to C in an aspect of thermodynamics. As described above, it is also required to improve strength, that is, tensile strength of springs. To improve the strength, an element essentially added is C. It is easy to add C. C improves strength of steel by improving precipitation strength together with other added alloy elements. However, when adding C simultaneously with a large amount of Si in an alloy, due to similar thermodynamic actions of C and Si, C and Si compete for a place, thereby generating a decarburization phenomenon where C is removed from the alloy.
As steel for spring with Si, there is SAE9250. Since a content of Si in the steel for spring is 1.8 to 2.0 wt %, a surface decarburization phenomenon of C from the steel becomes more serious. As a result, a fatigue life of the steel is decreased due to a surface-carburized layer in such a way that it is difficult to use the steel for a spring.
To solve such problems, Japanese Patent Application Nos. 1998-110247 and 1996-176737, Korean Patent Application No. 1997-0073576, and Korean Patent Laid-Open Publication No. 1999-0048929 disclose high tensile spring steel in which an overall amount of carbon is reduced and Ni is added to prevent an existence of a decarbonized portion on a surface, an amount of Si is more increased to restore a decrease in strength due to the decrease of the carbon amount, and Mo is additionally added in such a way that maximum designed toughness is increased to 1200 MPa.
However, in the case of the conventional steel, since the amount of Si is increased to improve yield strength and a deformation resistance in an aspect of alloy design, Si segregation occurs when continuously casting. Since the Si segregation is generally formed in a center of a steel wire rod, the occurrence of the segregation causes generation of ferrite in such a way that an nonuniformity of a central microstructure is caused, thereby generating a wide range of a change in properties and deteriorating toughness of a spring.
Also, since the conventional high stress steel contains a large amount of an alloy element, manufacturing costs are increased. In addition, due to the large amount of the added alloy element, though a steel wire rod is slowly cooled down at a relatively low speed when manufacturing the steel wire rod, there is generated a low temperature structure such as a composite structure of bainite and martensite. When the low temperature structure occurs while manufacturing a steel wire rod, a problem may be caused in processing in a latter process. That is, the low temperature structure such as bainite or martensite has high hardness due to internal toughness generated in transformation. The low temperature structure make it difficult peeling or shaving the steel wire rod to control diameter of the steel wire rod or modify surface quality before forming a spring using the steel wire rod. Accordingly, to smoothly peel or shave, a heat treatment such as a softening heat treatment is performed on the steel wire rod, which causes additional increase of manufacturing costs and deterioration of workability.
In addition, generally, since strength and toughness are opposite concepts to each other, it is difficult to provide strength and toughness at the same time. That is, generally, to improve strength of a spring, it is essential to form a rigid structure such as martensite or bainite in a steel wire rod. However, since being brittle, the rigid structure such as martensite or bainite has poor impact toughness.
As described above, a spring requires high strength to provide high permanent deformation resistance and fatigue strength and high toughness in addition to the high strength. Up to now, steel for a spring, which has both of high strength and high toughness, has not yet been developed. Also, since a low temperature structure occurs in a portion of the steel for a spring, spring custom company has to perform a softening heat treatment.
An aspect of the present invention provides a steel wire rod for a high strength and high toughness spring, which has excellent cold workability in a latter process, and a method of manufacturing the steel wire rod.
An aspect of the present invention also provides a method of manufacturing a high strength and high toughness spring by using the steel wire rod.